It is in general inevitable that impurities from a galvanizing apparatus, an electrode, galvanizing bath materials and a steel sheet to be electro-galvanized are entangled into a galvanizing bath during electrogalvanizing operations of the steel sheet. Impurities thus mixed into the galvanizing bath not only causes degradation of the surface quality of the produced galvanizing layer, but also exerts adverse effects on a chromate treatment to be applied thereafter. If, for example, a galvanizing bath contains Fe.sup.2+ mixed in it as impurities, formation of a chromate film on the galvanizing layer of an electro-galvanized steel sheet is seriously impaired in applying a chromate treatment as the next step, and hence, the amount of deposited chromate is largely reduced. If, furthermore, impurities such as Cu.sup.2+ and Ni.sup.2+ are contained in a galvanizing bath, the amount of chromate deposited onto the galvanizing layer of an electro-galvanized steel sheet is small in applying a chromate treatment. Consequently, an intensification of chromate treatment conditions, as described later, cannot increase at all the amount of deposited chromate. As a result, it is impossible to obtain a chromated electro-galvanized steel sheet having a satisfactory corrosion resistance after chromating.
For the purpose of preventing impurities from coming into a galvanizing bath, or removing impurities from a galvanizing bath, it has been usual practice to apply a closer control over impurities in a galvanizing bath, to employ a corrosion resistant material for the construction of a galvanizing apparatus, to remove such impurities as cadmium, lead and copper dissolved in a galvanizing bath by substituting zinc for such impurities through a treatment of the galvanizing bath with zinc powder, or, to cause precipitation of such impurities as copper by suspending an iron plate in a galvanizing bath.
On the other hand, a method for intensifying chromating conditions is conventionally known, which comprises increasing the amount of deposited chromate by increasing the amount of free acid in a chromating bath, with a view to imparting a satisfactory corrosion resistance to an electro-galvanized steel sheet of which the galvanizing layer has been degraded by impurities in the galvanizing bath. The chromating bath in this method has however a strong pickling action because of its increased free acid. This method is therefore defective in that the formation of a chromate film becomes nonuniform or the increased dissolution of zinc into the chromating bath accelerates the degradation of the chromating bath. Even by such an intensification of chromating conditions, therefore, the time before occurrence of white rust in a salt spray test, for example, is not extended so much, and an improvement of the corrosion resistance of a chromated electro-galvanized steel sheet cannot be expected.
In all cases, these conventional measures to prevent impurities from coming into a galvanizing bath, to remove impurities from a galvanizing bath and to intensify chromating conditions are only passive actions aiming at preventing the adaptability to chromating of an electro-galvanized steel sheet from being impaired by impurities mixed in a galvanizing bath. These measures cannot therefore be positive actions imparting a higher corrosion resistance to an electro-galvanized steel sheet by improving its adaptability to chromating.
In view of the foregoing, the following methods have so far been proposed:
1. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added Mo and W (refer to the Japanese Patent Publication No. 25,245/71); PA0 2. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added Co, Mo, W and Fe (refer to the Japanese Patent Publication No. 16,522/72); PA0 3. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added Co, Mo, W, Ni, Sn, Pb and Fe (refer to the Japanese Patent Publication No. 19,979/74); PA0 4. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added 0.05 - 0.3 g/l Cr.sup.6+ (refer to the Japanese Patent Provisional Publication No. 84,040//73); and PA0 5. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added 0.5 - 1.5 g/l Zr (refer to the Japanese Patent Publication No. 18,202/70). PA0 a. A process which comprises: subjecting a steel sheet to an electro-galvanizing treatment in a Zn-based acidic galvanizing bath containing at least one additive selected from the group consisting of: PA0 b. A process which comprises: subjecting a steel sheet to an electro-galvanizing treatment in a Zn-based acidic galvanizing bath containg one additive selected from the group consisting of: PA0 c. A process which comprises: subjecting a steel sheet to an electro-galvanizing treatment in a Zn-based acidic galvanizing bath containing at least one additive selected from the group consisting of: PA0 1. A conventional acidic galvanizing bath containing Zn only; PA0 2. A Zn-based acidic galvanizing bath containing 50 -10,000 ppm Co; and PA0 3. A Zn-based acidic galvanizing bath containing PA0 1. Cr.sup.3+ and Cr.sup.6+ PA0 2. Sn and In PA0 1. Co PA0 2. Cr.sup.3+ and Cr.sup.6+ PA0 3. Zr
All these methods (1) to (5) have an object to improve the quality of the galvanizing layer itself of an electro-galvanized steel sheet. The adaptability to chromating of an electro-galvanized steel sheet is not therefore improved by any of these methods, thus leading to no improvement in the corrosion resistance of the electro-galvanized steel sheet after a chromate treatment. Method (4) above (Japanese Patent Provisional Publication No. 84,040/73), in particular, has a drawback in that, because of the addition of Cr.sup.6+, hydrogen is absorbed into the steel sheet in the initial stage of electro-galvanizing, and this impairs the coating adhesion.
With a view to improving the coating adhesion in said method (4) (Japanese Patent Provisional Publication No. 84,040/73), a process has been proposed (refer to Japanese Patent Provisional Publication No. 98,337/74), which comprises subjecting a steel sheet to a first electrogalvanizing treatment in a galvanizing bath containing Zn only, to form a very thin pure zinc galvanizing layer of a thickness of at least 1 .times. 10.sup.-3 .mu., in practice of about 0.1.mu., on the surface thereof; and then, subjecting said electro-galvanized steel sheet with said pure zinc galvanizing layer formed thereon to a second electrogalvanizing treatment in a Zn-based galvanizing bath containing Cr.sup.6+. In this process, however, the galvanizing film formed on the steel sheet mostly comprises a galvanizing layer formed in a galvanizing bath containing Zn and Cr.sup.6+. A product obtained by this process is therefore only a steel sheet having a galvanizing layer formed in a galvanizing bath containg Zn and Cr.sup.6 +, after a primer treatment.
In view of the foregoing, a process for manufacturing a chromated electro-galvanized steel sheet has been proposed (refer to the Japanese Patent Provisional Publication No. 102,538/75), with a view to increasing the amount of deposited chromate film by the improvement of the adaptability to chromating of an electro-galvanized steel sheet and thus to improving the corrosion resistance of the electro-galvanized steel sheet after chromating, said process comprising: electro-galvanizing a steel sheet in a Zn-based acidic galvanizing bath containing an additive selected from the group consisting of:
a. Cr.sup.3+ -- 50 - 700 ppm, PA1 b. Cr.sup.6+ -- 50 - 500 ppm, and PA1 c. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which PA1 Cr.sup.6+ being 500 ppm at the maximum; PA1 i. Cr.sup.3+ -- 50 - 700 ppm, PA1 ii. Cr.sup.6+ -- 50 - 500 ppm, PA1 iii. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which PA1 Cr.sup.6+ being 500 ppm at the maximum, PA1 iv. In -- 10 - 3,000 ppm, and PA1 v. Zr -- 10 - 2,500 ppm; and: PA1 vi. Co -- 50 - 10,000 ppm, PA1 i. Cr.sup.3+ -- 50 - 700 ppm, PA1 ii. Cr.sup.6+ -- 50 - 500 ppm, and PA1 iii. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which PA1 Cr.sup.6+ being 500 ppm at the maximum; PA1 iv. Sn -- 10 - 5,000 ppm, and PA1 v. In -- 10 - 3,000 ppm, PA1 i. In -- 10 - 3,000 ppm, and PA1 ii. Sn -- 10 - 5,000 ppm, PA1 i. Co -- 50 - 10,000 ppm; PA1 ii. Cr.sup.3+ -- 50 - 700 ppm, PA1 iii. Cr.sup.6+ -- 50 - 500 ppm, PA1 iv. Cr.sup.3+ and Cr.sup.6+ 13 50 - 700 ppm, in which PA1 Cr.sup.6+ being 500 ppm at the maximum, and PA1 v. Zr -- 10 - 2,500 ppm, PA1 a. Cr.sup.3 + -- 50 - 700 ppm, PA1 b. Cr.sup.6+ -- 50 - 500 ppm, PA1 c. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which PA1 Cr.sup.6+ being 500 ppm at the maximum, PA1 d. Sn -- 10 - 5,000 ppm, and PA1 e. In -- 10 - 3,000 ppm, PA1 1. Cr.sup.3+ -- 50 - 700 ppm, PA1 2. Cr.sup.6+ -- 50 - 500 ppm, PA1 3. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which Cr.sup.6+ being 500 ppm at the maximum, PA1 4. Sn -- 10 - 5,000 ppm, and PA1 5. In -- 10 - 3,000 ppm, PA1 1. Cr.sup.3+ -- 50 - 700 ppm, PA1 2. Cr.sup.6+ 50 - 500 ppm, PA1 3. Cr.sup.3+ and Cr.sup.6+ 50 - 700 ppm, in which Cr.sup.6+ being 500 ppm at the maximum, PA1 4. Sn -- 10 - 5,000 ppm, and PA1 5. In -- 10 - 3,000 ppm, PA1 In the galvanizing layer of an electro-galvanized steel sheet, formed in a galvanizing bath containing Cr.sup.3+ and/or Cr.sup.6+, Cr.sup.3+ and CR.sup.6+ are chemically absorbed in the form of oxides and/or hydroxides of Cr into said galvanizing layer, which are estimated to serve as nuclei on the formation of a chromate film and promote the growth of the chromate film. PA1 A cr.sup.3+ content of over 700 ppm in a galvanizing bath is not desirable because of a portion remaining undissolved in the galvanizing bath. Also, a Cr.sup.6+ content of over 500 ppm in a galvanizing bath impairs the adhesion of zinc to steel sheet and produces irregularities in the galvanizing layer, thus giving an unfavorable external appearance to the electro-galvanized steel sheet. Furthermore, an excessive content of Cr.sup.6+ in a galvanizing bath inhibits formation of a galvanizing film. PA1 Steel sheets were tentatively electro-galvanized, one in a conventional acidic galvanizing bath based on zinc sulfate (ZnSO.sub.4) and added with ammonium chloride and pH buffer, and the second one in an acidic galvanizing bath prepared by adding tin sulfate (SnSO.sub.4) into said conventional bath, and the third one in an acidic galvanizing bath prepared by adding indium sulfate (In.sub.2 (SO.sub.4).sub.3) into said conventional bath, at a current density of 45 A/dm.sup.2, so as to give an amount of deposited zinc of 20 g/m.sup.2 ; and then subjected to a chromate treatment by dipping said electro-galvanized steel sheets in a commercial reactive-type chromating solution. Measurement of the natural electric potential on said chromated electro-galvanized steel sheets have shown that the chromated electro-galvanized steel sheet treated in the Sn-containing or in the In-containing bath has a far higher (base) natural electric potential than in that treated in the galvanizing bath containing none of these elements. There was almost no difference in the natural electric potential between the electro-galvanized steel sheet treated in the Sn-containing galvanizing bath and that treated in the In-containing bath, the former showing a slightly higher (base) natural electric potential. These results indicate that the surface of a galvanizing layer formed in a galvanizing bath containing Sn or In is more activated (base) than that of a galvanizing layer formed in a galvanizing bath containing none of these elements. PA1 Furthermore, the amount of deposited chromate film of the above-mentioned chromated electro-galvanized steel sheet treated in the Sn-containing galvanizing bath, as measured by fluorescent X-ray, was 2.5 times that treated in the galvanizing bath containing none of Sn and In, and that of the chromated electro-galvanized steel sheet treated in the In-containing galvanizing bath was 2.1 time the latter. This permitted confirmation of the fact that the surface activation effect by Sn and/or In causes increase in the amount of deposited chromate film on the surface of a galvanizing layer. PA1 However, an Sn content of over 5,000 ppm in a galvanizing bath causes precipitation of an undissolved portion in the galvanizing bath. In spite of the deposition of zinc, the impaired adhesion of zinc to a steel sheet prevents the formation of a galvanizing film. In the case of an Sn content of under 10 ppm, on the other hand, there is observed no improvement in the adaptability to chromating of a galvanized steel sheet. PA1 Also, an In content of over 3,000 ppm in a galvanizing bath, posing no problems in the formation of a galvanizing layer, the adhesion of the galvanizing layer and the adaptability to chromating of an electro-galvanized steel sheet, causes formation of deposits on a galvanizing electrode, thus making it difficult to carry on galvanizing operations. An In content of under 10 ppm, on the other hand, brings about no improvement in the adaptability to chromating of an electro-galvanized steel sheet. PA1 Preferable additives to add Sn in a galvanizing bath include such water-soluble compounds as stannous sulfate, stannic sulfate, stannous chloride and stannic chloride, and preferable additives to add In include such water-soluble compounds as indium sulfate and indium chloride. PA1 i. Co -- 50 - 10,000 ppm; PA1 ii. Cr.sup.3+ -- 50 - 700 ppm, PA1 iii. Cr.sup.6+ -- 50 - 500 ppm, PA1 iv. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which Cr.sup.6+ being 500 ppm at the maximum, and PA1 v. Zr -- 10 - 2,500 ppm, PA1 i. Co -- 50 -0 10,000 ppm; PA1 ii. Cr.sup.3+ -- 50 - 700 ppm, PA1 iii. Cr.sup.6+ -- 50 - 500 ppm, PA1 iv. Cr.sup.3+ and Cr.sup.6+ -- 50 - 700 ppm, in which Cr.sup.6+ being 500 ppm at the maximum, and PA1 v. Zr -- 10 - 2,500 ppm, into an acidic galvanizing bath having the same chemical composition as that of the galvanizing bath used for the first electro-galvanizing treatment for forming the first galvanizing layer in the first process of the present invention.
and then subjecting said electro-galvanized steel sheet to a chromate treatment. According to this method, the time before occurrence of white rust is largely extended, but sufficiently satisfactory results are not as yet available in terms of the time before occurrence of red rust.
Under these circumstances, we have previously proposed the following three processes for manufacturing a chromated electro-galvanized steel sheet having a largely improved corrosion resistance (especially the time before the occurrence of red rust), said processes permitting increase in the amount of deposited chromate film in a high-speed line treatment:
to form a galvanizing layer on the surface thereof; and then, subjecting said electro-galvanized steel sheet with said galvanizing layer formed thereon to a conventional chromate treatment (refer to the Japanese Patent Application No. 8,627/75);
and at least one additive selected from the group consisting of:
to form a galvanizing layer on the surface thereof; and then, subjecting said electro-galvanized steel sheet with said galvanizing layer formed thereon to a conventional chromate treatment (refer to the Japanese Patent Application No. 8,626/75); and
to form a galvanizing layer on the surface thereof; and then, subjecting said electro-galvanized steel sheet with said galvanizing layer formed thereon to a conventional chromate treatment (refer to the Japanese Patent Application No. 32,250/75).
All the above-mentioned methods (a), (b) and (c) have an object to form a galvanizing layer excellent in the adaptability to chromating on the surface of a steel sheet by adding at least one additive into an acidic galvanizing bath, and enable to obtain an electro-galvanized steel sheet excellent in the adaptability to chromating. However, in terms of the corrosion resistance of the galvanizing layer itself on an electro-galvanized steel sheet before chromating (hereafter referred to as the "bare corrosion resistance"), an electro-galvanized steel sheet produced by any of methods (b) and (c) mentioned above is not always superior to an electro-galvanized steel sheet having a galvanizing layer obtained by a conventional electro-galvanizing treatment with the use of a galvanizing bath not containing such additives as mentioned above (hereafter referred to as the "pure zinc galvanizing layer"). For example, a galvanizing bath, when containing too much Cr with a view to largely improving the adaptability to chromating of an electro-galvanized steel sheet, causes precipitation of much Cr on the interface between the steel sheet and the galvanizing layer thereof, and thus impairs the adhesion of the galvanizing layer. A galvanizing bath containing Sn, on the other hand, tends to cause pinholes in the resulting galvanizing layer, which may lead to drawbacks such as the degradation of the bare corrosion resistance of the electro-galvanized steel sheet thus obtained.
The amount of deposited zinc layer is in general smaller in an electro-galvanized steel sheet than in a hot-dip galvanized steel sheet. In terms of the overall corrosion resistance of a chromated electro-galvanized steel sheet as a whole after a chromate treatment, however, the ratio of the corrosion resistance of the chromate film to the overall corrosion resistance reportedly accounts for about 50%. The effect of the chromate film of an electro-galvanized steel on the overall corrosion resistance is therefore greater than that in a hot-dip galvanized steel sheet, and plays a very important role on the corrosion resistance. More specifically, the overall corrosion resistance of a chromated electro-galvanized steel sheet is based on the cooperation of the corrosion resistance of the galvanizing layer thereof as defined as the bare corrosion resistance and the corrosion resistance of the chromate film thereof. Even if the galvanizing layer has a low bare corrosion resistance, therefore, a chromated electro-galvanized steel sheet shows an excellent corrosion resistance as a whole, when the galvanizing layer has a high adaptability to chromating. In contrast, when the corrosion resistance of the chromate film is degraded with time, or when the galvanizing layer is exposed by a damage to the chromate film, a low bare corrosion resistance of the galvanizing layer accelerates the occurrence of rust and leads to a lower overall corrosion resistance of a chromated electro-galvanized steel sheet as a whole.
As is clear from the foregoing, a chromated electro-galvanized steel sheet is required to have an excellent bare corrosion resistance of the galvanizing layer thereof as well as an excellent corrosion resistance of the chromate film thereof. Degradation of any of these corrosion resistances impairs the overall corrosion resistance of said chromated electro-galvanized steel sheet as a whole.
A chromated electro-galvanized steel sheet obtained by method (a) mentioned above has a higher bare corrosion resistance and a considerably improved corrosion resistance after a chromate treatment under the cooperative effects of such additives as Co, Cr, In and Zr in the galvanizing bath, as compared with a chromated electro-galvanized steel sheet obtained by any of methods (b) and (c) mentioned above. It is however inevitable that the galvanizing layer of a chromated electro-galvanized steel sheet obtained by method (a), which contains Co, has a smaller amount of deposited chromate film as compared with a chromated electro-galvanized steel sheet having a galvanizing layer not containing Co, and the quality of the former may therefore be degraded with time.
More specifically, an electro-galvanized steel sheet has usually a press formability different from that of an ordinary cold rolled steel sheet not galvanized, and the press formability of an electro-galvanized steel sheet depends also on the application of a chemical treatment and the type thereof. In addition, an electrogalvanized steel sheet is characterized in that it has a lower stretch formability but a higher deep-drawing formability.
Chromated electro-galvanized steel sheets with amounts of deposited chromate film of 40mg/m.sup.2 and 9mg/m.sup.2, respectively, were manufactured by chromating electrogalvanized steel sheets each having a conventional galvanizing layer not containing any additional element in a commercially available conventional chromating solution. On these steel sheets, the corrosion resistance and the deep-drawing formability were investigated at moments immediately after the manufacture and after a six-month in-door holding in a packaged form. As a result, almost no difference was observed in the corrosion resistance between the two sheets both immediately after the manufacture and after the lapse of six months. With regard to the deep-drawing formability, however, although there was no difference between the two sheets immediately after the manufacture, a serious degradation was observed in the one with an amount of deposited chromate film of 9mg/m.sup.2 after the lapse of six months.
It was thus found that, depending upon the amount of deposited chromate film, the deep-drawing formability of chromated electro-galvanized steel sheets show a difference with time. The reasons are not clearly known, since the press formability of an electron-galvanized steel sheet shows complicated behaviors depending on the presense of a chemical treatment, the type of the chemical treatment applied and the lapse of time, unlike that of a cold rolled steel sheet not galvanized. However, it is at least evident that the amount of deposited chromate film is significant.
It may be concluded from these facts that the best way for preventing the secular degradation of the deep-drawing formability of a chromated electro-galvanized steel sheet is to increase the amount of deposited chromate film.
As mentioned above, it is particularly necessary for a chromated electro-galvanized steel sheet to be excellent in the bare corrosion resistance of the galvanizing layer and the corrosion resistance of the chromate film. It should furthermore have an amount of deposited chromate film sufficient to prevent the secular change of the deep-drawing formability thereof. However, a chromated electro-galvanized steel sheet provided with all such performances and a manufacturing process thereof have not as yet been proposed.